Note: Descriptions are shown in the official language in which they were submitted.
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DEVICES COMPRISING A BOO~ OF A Fe-Ni rllAGNETIC ALLOY
Technieal Eield
The invention pertains to devices comprising a
body of a Fe-Ni magnetic alloy.
_ ground of the Invention
Maynetically soft materials, i.eO, materials
which typically exhibit macroscopie ferromagnetism only
when a magnetic field is applied, find applieation in a
great variety of technologieal fields. Exemplary uses are
in heavy-c-lrrent engineering, transduetor cores, relays,
inductance eoils, transformers, and variable reluetanee
devices. Although many materials are soft magnets~ this
invention is coneerned only with magnetieally soft iron-
niekel (~e-Ni) alloys, and in partieular, Fe-rieh
essentially ferritie alloys, and the diseussion will be
restrieted aeeordingly.
The Fe-Ni alloy system offers a large number of
teehnieally important magnetieally soft eompositions,
typieally having eompositions in the range 30-80 weight
pereent Ni. See for instanee C. W. Chen, Magnetism and
~letallurgy of Soft Magne-tie ~laterials, North-E3011and
Publishing Co., 1977, page 389. Alloys in this
eompositional range have the austenitie (faee-eentered
eubie, fee) erystal structureO M. Hansen, Constitution of
Binary Alloys, 2nd ed., McGraw-Hill, (1958), pp~ 677-6~4.
In Fe-Ni alloys within the compositional range
from O to about 20 weight percent ~i, the body centered
eubie (bec) lattice configuration prevails, and within the
range of from about ?.0 to about 30 percent Ni, after normal
eooling from they-region to room temperature, a two-phase
strueture eontaining both a bee and an fee phase typieally
exists.
As a general rule, for soft magnetie materials
the final produet should be a single-phase solid solution
in the equilibrium state, (W. Chen, op. cit. page 267~.
-- 2 --
In agreement with this rule the above two-phase region,
i.e., the region from about 20 to 30 percent Ni, is usually
not of magnetic interest. However, alloys near 30 percent
Ni in the single-phase fcc region find application as tem-
perature compensators.
In the prior art, Fe-Ni alloys having the compositional
range 0 to 20 weight percent Ni have not found significant
use, although their properties have been measured and pub-
lished. See, for instance~ R. M. Bozorth, Ferromagnetism,
Van Nostrand, 1951, especially pp. 102-119, and G. Y. Chin
and J~ H. ~ernick, Ferromagnetic Materials, Vol. 2,
E. P. Wohlfar~h, editor, North-Holland Publishing Co.,
(1980), especially pp. 123-168~ The neglect of alloys
in this compositional range can be explained by their
technologically relatively unattractive magnetic
characteristics, such as, for instance, their relatively
low maximum permeability and relatively high coercive
force, as exemplified by the prior art data referred to
above. However, alloys in this composi~ional range have
low material costs, and fur~hermore, supplies for Fe
and Ni are substantially assured. Thus, Fe-Ni alloys
containing less than abcut 20 weight percent Ni could
be of considerable commercial value if their magnetic
properties could be sufficien~ly improved.
An e~tablished soft magnetic material, used for
instance as a ring armature in telephone receivers~ is
2V-Permendur ~49 percent Fe, 4g percent Co, 2 percent V).
But the high cost and uncertain supply status o~ Co make
development of a Co-free substitute material for this and
other high-Co alloys desirable.
Summary of the Invention
According to one aspect of the inven~ion there i~
provided a device comprising a hody of a magnetically soft
Fe-Ni alloy, CHARACTERIZED IN THAT the alloy has a; a Ni
content in the range of about 4 to about 16 weigh~ percent,
and a multiphase structure, and b) a maximum permeability
~m at least as large as
2~
- 2a -
the value given by the expression l.S[25(16 x)2]G/Oe,
w'i~ere "x" is equal to the weight percent of Ni~
According to another aspect of the invention there is
provided a device comprising a com.ponent whose position
. is dependent on strenyth or direction o' a magnetic field,
the component comprising a body of a magnetically soft
Fe-Ni alloy, CHARACTERIZED IN THAT the alloy has a) a Ni
content in the range of about 4 to about 16 weight percent,
and a multiphase structure, and b) a maximum permeability ~m
at least as large as the value given by the ex~ression
1.5[25(16-x)2]G/Oe, ~.~ere "x" i5 equal to the ~7eîght percent o~ Ni.
According to the invention, improved magnetically soft
Fe-Ni alloys with a Ni content in the range from ab~ut 4
to about 16 weight percent~ preferably from about 6 to
about 12 weight percent are realized. In particular, the
inventive alloys have a maximum permeability ~m at least
equal to the value given by the expression 1.5[25(16-
,, .
x)2](1.257 10 6)~b/A~ m {1.5[2s(16-x)2]~/Qe}, and
typically have a coercive force Hc at most equal to the
value given by the expression .7(79.6)[o~5(l+o~x)~Alm
{.7[0.65(1+0.6x)]0e}, with "x" being the weight percent of
Ni. Typicall~, the alloys also exhibit a saturation
induction Bs of at least about 2 Wb/m2 {20 kG}, and a
~aximum incremental permeability ~, measured with an
applied a.c. field ~ of about (79~6) (0.005)A/m
{0.005 Oe}, of at least about 150(1.257- 10-6)Wb/A- m
1() {150 G/Oe}. Also, the alloys typically exhibit a yield
strength to 0.2~ offs~t o~ at least about 40- 6.895 106 Pa
{~0~103psi}. The inventiv~ alloys are fabricated by a
process comprising alow temperature anneal in the ~+Y
region of the phase diagram, preferably at a temperature
within the range ~efined by the expression [750-
17x]C+25C, in which "x" represents weight percent ~i.
The inventive alloys typically contain only Fe,
Ni and "steelmaking additives" in individual amounts
greater than about 0.5 percent by weight. By "steelmaking
additives" we mean those elements that have been added in
steelmaking for purposes of de-sul~urization, de-
carburization, de-oxidation, and the like, and which may be
present in the starting materials for the inventive alloy
in a concentration in excess of 0.5 percent by weight, but
~5 typically less than about 1 percent by weight. Examples of
such elements are Mnl ~1, Zr and Si~ However, in preferred
alloys "steelmaking additives" do not exceed 0.5 percent by
weight individually.
Preferred inventive alloys typically do not
contain additives and impurities in a combined a~ount
greater than about 1 percent by weight, preferably not
greater than 0O5 percent, and individual additives and
impurities typically are present only in amounts less than
about 0.5 percent by weight, preferably less than
0.2 percent. Carbon, nitrogen~ oxygen, sulfur and
phosphorus typically are present only in amounts less than
0.1 percent by weight, preferably less than 0.05 percent.
The above combination of advantageous magnetic
and mecharlical properties permits use of bodies comprising
an inventive alloy in device applicationsO For ins~ance, a
body comprislng an alloy according to the invention
typically can advantageously be incorporated in-to a device
comprising a component whose position is dependent on
strength or direction of a magnetic field, and is
particularly advantageously incorporated into an electro-
acoustic transducer, e.g., into such a transducer contained
in a telephone receiver. And alloys according to the
invention typically can advantageously be used to replace
some high-cost prior art alloys, e.g., 2V-Permendur, in
devices such as telephone receivers.
Brief Description of the Drawings
-
FIG. 1 shows maximum permeability, coercive
force, saturation induction, and resistivity of prior art
alloys having ~i content between about 4 and about 16
weig}-~t percent;
FIGo 2 shows B-H loops of a Fe-12Ni alloy
according to the invention;
FIGS. 3 and 4 show maximum permeability and
coercive force of a Fe-6Ni alloy and a Fe-12Ni alloy,
respectively, as a function of heat treating time and
temperature;
FIGS~ 5 and 5 present data on the incremental
permeability of 2 alloy compositions according to the
invention as a function of biasing field; and
FIG. 7 schematically illustrates in cross-
sectional view a device comprising a magnetic body
according to the invention~ In particular, it illustrates
a U-type telephone receiver.
Detailed Description
Fe-Ni alloys with a Ni content in the range from
about 4 to about 16 weight percent can be processed to have
improved magnetic properties that typically make such
alloys useful as magnetically soft components in devices.
In particular, alloys according to the invention have
maXililUm permeability ~m that is more than about 50
percent, preferably more than 100 percent, greater than
that of prior art E`e-Ni alloys of t~e same Ni content and
typically have coercive force Hc at least a~out 30%,
preferably 50~, less than that of such prior art alloys.
Flurthermore, the inventive alloys exhibit values of
saturation induction Bs~ incremental permeability Q~ ,
electric~l resistivity p, and yield strength that are
similar to, and in the case of ~ 9 significantly higher
than, those of prior art Fe-Ni alloys of the same Ni
content. The inventive alloys typically can advantageously
be employed in devices comprising a body of a magnetically
soft metallic alloy, exemplified by devices comprising a
cornponent whose position is dependent on strength or
direction of a magnetic field. Among such devices are
electro-acoustic transducers, such as, Eor instance, those
used in U-type telephone receivers.
Alloys according to the invention typically do
not contain any elements other than Fe and Ni in individual
amounts greater than about 0.5 percent by weight,
preferably 0.2 percent, except for "steelmaking additives"
such as Mn, ~1, Zr and Si, as was pointed out above. In
preferred alloys "steelmaking additives" also do not exceed
0.5 percent by weight individually. Also, preferred alloys
according to the invention typically do not contain
additives and impurities in a combined amount greater than
about 1 percent by we~ght, preferably less than
0.5 percent. Examples of elements that can be present
either as additives or as impurities are Mn, Al, Zr, Si,
Cu, Cr, Co, ~o, Ti, and V. The elements C, N, O, S, and P
typically are present as deleterious impurities, and are to
be present individually in amounts less than about
0.1 percent by weight, preferably less than 0.05 percent,
in order to achieve superior magnetic and mechanical
properties.
The inventive alloys typically possess a multi-
phase structure, comprising ferritic (bcc, ~-phase),
L6
austenitic (fcc~y-phase)~ and rnartensitic (bcc, ~'-phase)
constituents. The distribution of phases present in any
particuïar alloy depends on composition and heat treatment.
The heat treatment typically comprises a "low temperature"
annealing step at a temperature within the(~+y) two-phase
region oE the Fe-Ni phase diagram. Such treatment
typically results in relief of internal stress and in
annealing-out of defects, and consequently in slight
mechanical softening, as well as in pronounced magnetic
"softening'l. Prolonged heat treatment, however, leads to
the formation of an excessive amount of undesirable
retained austenite, which results in deterioration of the
soft magnetic properties, especially in alloys with higher
Ni-content, as will be demonstrated below.
Alloys according to the invention can, for
instance, be prepared by vacuum induction-melting of Fe and
Ni or their alloys in the appropriate amounts to yield the
desired nominal alloy composition, casting ingots from the
melt, "soaking" the ingot for an extended period at
elevated temperature, for instance at about 1250C for
about 4 hours, followed by an appropriate hot-forming
operation and air cooling. The resul-ting material is then
typically further processed to yield a component of the
desired shape. The metal forming steps typically are
followed by heat treatment, which typically comprises an
extended anneal at a temperature in the ~region of the Fe-
Ni phase diagram, e.g., about 2 hours at about 1000C,
carried out in a protective atrnosphere, e.y., in H2,
followed by an air cool. This in turn is typically
followed by the above-described "low-temperature" heat
treatment in the two-phase region o~ the phase diagram,
which is typically also carried out in a protective
atrnosphere, e.g., in Ar, H2, or N2.
It will be understood that the details of the
heat treatment can be varied, provided the -treatment
results in a rela-tively strain- and defect-free multi-
phase material that does not contain excessive a~ounts of
2~i
retained austenite.
Although annealiny at substantially any
temperature within the (c~ region of the phase diagram
will result in decreaserl internal stress and in a reduced
5 concentration of defects, a preferred temperature range for
the low temperature heat treating step is given by the
following expression:
heat treatment temperature ~ [750 - 17x]C~25C
In this expression, as well as elsewhere in this
10 application, "x~' represents the weight percent Nio The
"low-temperature" heat treatment time, yielding, for
instc,nce, maximum IJm~ is typically dependent on temperature
and on alloy composition, as will be si~own below.
Establishment of the appropriate heat treatment time thus
15 typically requires a minor amount of experimentation.
FIG. 1 shows typical prior art values of maximum
permeability ~m as curve 10, coercive force Hc as curve 11,
saturation induction Bs as curve 12, and electrical
resistivity P as curve 13, as a function of Ni content.
20 Over the compositional range of interest to this invention,
i.e., for about ~1-16 wei~ht percent Ni, the prior art
values of ~m can be approximated by the expression
25(16-x)2 (1.257 10 6)Wb/A m
[25(16~x)2 ~:;/Oe], and of Hc by the expression
25 0.65(1~0.6x)(79.6)A/m [0.65(1~0.6x)0e] . These as well as
all other values of magnetic and mechanical properties
citecl herein are understood to be room-temperature values.
Alloys according to the invention have
substantially improved maximum permeability and coercive
30 field over prior art alloys~ ~m being typically increased
by at least about 50 percent, preferably 100 percent, and
Hc being typically decreased by at least about 30%,
preferably by at least about 50~x. Inventive alloys
therefore have llm at least equal to the value given by the
35 expression 1.5[25(16-x)2] (1.257 10 6)l~`1b/'A m {1.5[25(16-
3~
- 8 -
x)2jG/Oe}, preferably 2[2S(l~-x)2] (1~257 10-6)Wb/A m
{2[25(16-~)2]G/Oe}, and TlC at most equal to the value of
the expression 0.7(79~6)[0.~5(1+0.6x)]~/m
{0.7[0.65(1+0.6x)1Oe}, preferably
0.5(79.6)[0.65(1+0.6x)]~/m {0.5[0.65(11-0.~x)]Oe}.
Yurthermore, such alloys exhibit a saturation induction Bs
of at least about 2 ~b/rll2 (2n kG), a maximum incremental
permeability ~ of at least about 150(1.257 ~10-~)Wb/A m
(150 G/Oe), preferably 200(1.257 10-6)Wb/A m (200 G/Oe),
when measured with an applied a.c. magnetic field of about
79.6(0.005)A/m (0.005 Oe), and a yield strength to 0.2
percent offset of at least about 40 ~.895 106 Pa
(40 103 psi).
As had been stated ahove, alloys according to the
invention comprise about ~-15 percent by weight of Ni, with
the preferred range being from about 6 percent to about
12 percent. The lower limit is dictated by strength and
resistivity considerations, since heat-treated Fe-Ni alloys
containing less than about 4 percent Ni typically are too
soft and have too low resistivity for device applications.
The upper limit of Ni content is dictated by coercive field
and permeability considerations, since in Fe-Ni alloys
containing more than about 16 percent Mi typically Hc is
too large and ~m and ~ too small for device applications
requiring a magnetically soft material. The range from 6-
12 percent by weight of Ni typically offers the most
advantageous combination of magnetic and mechanical
propertiest and is therefore preferred.
FIG. 2 illustrates some aspects of the changes
that take place in the magnetic properties of alloys
according to the invention when subjected to various heat
treatments, namely, the figure shows B-H loops of samples
of Fe-12Ni (i.e.~ an Fe-Ni alloy containing nominally
12 percent by weight of Ni). Curve 20 of FIG. 2 is
obtained with a sample that was annealed at about
~$~
_ 9
1000 degrees C (i.e. 9 in the ~region of the phase diagram)
for about 2 hours, followed by an air cool The resulting
martensitic structure is found to have a high density of
dislocations and point defects, a fine substructure, and
internal stress due to the rapid change in crystal
structure without significant long-range diffusion. These
structural features result in magnetic properties that make
the sample typically unsuitable for applications requiring
a maynetically soft material, as is revealed by the skewed
B-H loop. In particular, the sample has a relatively large
Hc, relatively small B, e.g., B25 [i.e., B at H=2.0- 103A/m
(25 Oe)] and relatively small ~m and A~. Curve 21 of
FIG. 2 is obtained after heat-treatment of a martensitic
sample within the low-temperature (a+y)two-phase region,
namely at about 550 degrees C for about 2 hours. Although
such heat treatment typically results in decomposition of
the alloy into a multi-phase structure (e.g.,~+Y+~'), it
results in significantly improved magnetic properties,
e.g., decreased Hc and increased B, ~m~ and ~
FIGS. 3 and 4 exemplify the dependence of
magnetic properties, in particular of ~ and Hc, on heat
treating time and temperature, for samples of Fe-fiNi
(FIG. 3) and of Fe-12Ni alloys (FIG. 4). Both alloys show
a rapid initial increase in ~m and decrease in Hc, with the
rate of change increasing both with temperature and with Ni
content. But whereas Fe-6Ni samples do not show any
"reversion" (i.e., excessive retained austenite formation)
after 8 hours at temperatures up to 650 degrees C, Fe-12Ni
samples show reversion for times greater than about
0.5 hours and 2 hours at 600 degrees C and 550 degrees C,
respectively, demonstrating that typically the annealing
and transformation rates increase with both temperature and
Ni content.
FIGS. 5 and 6 show the incremental permeabilities
~ of samples of Fe-6Ni (heat treated at 1000 degrees C for
2 hours and at 650 degrees C for 30 minutes) and of Fe-l~Ni
(1000 degrees C/2 hours and 550 degrees C/2 hours), as a
function of biasing field. The amplitude of the a.c.
measuring field, referred to as ~H, is 0.5 79O~ ~/m
~0.5 Oe) and 0.005 79.6 A/m (0.005 Oe) for FIGS. 5 and 6,
respectively. The maximum incremental permeability
decreases both with increasing Ni content and with
decreasing ~].
FIG. 7 schematically shows in cross~section an
example of a device that comprises a component whose
position is dependent on the strength or direction of a
magnetic field. In particular, the figure represents an
electro-acoustic transducer, and still more particularly, a
U-type ring-armature telephone receiver, as described for
instance by E. E. Mott and R~ C. Miner, Bell System
Technical Journal, vol. 30, pp. 110-140 (1951). Permanent
magnet 70, for example a Fe-Cr-Co magnet, provides a
biasing field in the air gap formed between pole piece 71,
which, for example, can be a body comprising a Fe--
~5Ni alloy, and one pole of 70. Armature ring 72,
typically comprising a magnetically soft alloy such as, for
instance, 2V-Permendur in a prior art device, or an Fe-Ni
alloy according to the invention, rests on non-magnetic
support 7~, and can be subjected to a time-varying magnetic
field by means of electrical induction coil 73. The
position of the armature 72 in the air gap is a function of
the strength and direction of the time-varying magnetic
field, resulting in movement of the armature 72 and of
diaphragm 75, attached to the armature 72, thereby creating
acoustic waves in a surrounding fluid medium, e.g., in air.
~lloys useful as armatures in telephone receivers must have
a large ~m~ large ~ at a high induction, and suitable
mechanical properties, namely high yield strength, and
alloys according to the invention typically do possess
these properties.
In addition to advantageous magnetic properties
and high yield strength, alloys according to the invention
and bodies produced therefrom also have other useful
mechanical properties. In particular, they are typically
31 ~9~ 6
-- 11 --
ductile, and are easy to process since they io not have
critical processing s-teps and are not subject to pronounced
work hardening during deformation.
In Table 1 is presented data on yield strength of
E`e-Ni alloys with and without low-temperature heat
treatment. The data shows that the anneal in the two-phase
region results in a relatively minor decrease in yield
strength.
In Table 2 is presented typical magnetic data and
the room-temperature resistivity for two compositions of
inventive alloys. A typical heat treatment for the Fe-6Ni
samples is 1000C/2 hours + 650C/30 minutes, and for the
Fe-12Ni samples is 1000C/2 hours + 550C/2 hours.
And in Table 3 we represent exemplary
measurement results on armature rings made from inventive
alloys.
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In the first column of Table 3 are listed the alloy
compositions, the annealing times and temperatures, and the
times, temperatures, and protective gas used for the low-
temperature two-phase anneal. ~s25i' refers to the magnetic
induction measured with an applied field of 25 79.h A/m
(25 Oe).
~ s can be seen from the data presented in
Table 3, the details of the heat treatment, especially of
the low-temperature treatmentl typically have a substantial
effect on the magnetic properties of the alloys, especially
on ~m For instance, the first-listed Fe 6Mi samp]e shows a
low ~m because the heat treatment time and temperature were
insufficient, as can also be verified from FIG. 3. Thusr
it is typically necessary to establish, for instance by
measurements such as those that lead to the data shown in
FIGS. 3 and 4, the relationship between alloy composition,
annealing temperature and time, and the relevant magnetic
properties. However, heat treatment of alloys according to
the invention is not limited to the exemplary sequences and
conditions disclosed above, and variations thereon will be
obvious to those skilled in the art.
